Magnesium alloys have become increasingly popular for their distinct characteristics. Magnesium is the lightest of all structural materials, and has excellent strength-to-weight ratio and stiffness. Also, magnesium has EMI shielding properties, and is thus widely used in electronic devices. For example, magnesium is now used in cellular phones, cameras, CD players and other handheld devices. Accordingly, die-casting machines have been adapted over the years to produce components in magnesium.
In the die-casting industry, there are generally two types of casting machines. The conventional die-casting machine has a moving platen and a fixed platen, the platens having complementary die portions. The moving portion is displaceable so as to form a cavity with its die portion and the die portion of the fixed platen. An injection unit is positioned in the fixed part and is supplied with molten medium from an injector unit so as to fill the cavity with the molten medium. The molten medium solidifies in the cavity in the shape thereof. The cast piece is then ejected from the cavity by the separation of the moving part from the fixed part. The injection unit is either directly positioned in a molten metal bath, in the case of a hot chamber die casting machine, or receives a molten metal supply from a surrounding furnace, in the case of a cold chamber die-casting machine.
In parting-line casting, a multiple-slide die-casting machine is used, and has two or more moving slides each having a die portion. The slides meet to form a die cavity from all of the die portions. The injection unit is generally positioned on the parting line between a pair of slides, hence giving it the name “parting-line injection/casting.” More complex parts can be cast in parting-line casting than in conventional casting. However, more complex motions are also involved. Typically, the slides are each displaceable. The die, resulting from the cooperation of the slides, and the injection unit must meet for the injection. Therefore, there must be some relative displacement between the injection unit and the slides.
One of the ways to perform the cooperation between the slides and the injection unit is to provide mobility to the injection unit. The injection unit has a portion thereof in a bath of molten casting medium from which it supplies the die. In the case of conventional magnesium die-casting, the bath of molten magnesium has a film of shielding fluid on a top surface thereof to reduce oxidation between the molten magnesium and the ambient oxygen. Displacements of the injection unit in the bath create an exposure of the molten medium, but the shielding film on the surface of the bath ensures that the molten magnesium is not overexposed to oxygen, or to air moisture. However, in previous attempts to cast pieces in magnesium in parting-line casting, because of the reactive nature of molten magnesium due to its relatively high melting point, the displacement of the injection unit in the molten bath of magnesium causes waves in the liquid metal. The waves disturb the shielding gas, and this results in oxidation, thereby necessitating frequent cleaning to remove the oxide build-ups.
Magnesium has a relatively high melting point, but a relatively low specific heat. There is a risk of solidification of the magnesium prior to the molten magnesium reaching the die. Therefore, temperature control is an important aspect of magnesium die-casting.
Also, molten magnesium is highly reactive and safety measures must be taken when magnesium is cast. For instance, extensive use of hydraulic fluid should be limited. It is desirable to adapt parting-line die-casting technology to magnesium, to enable the casting of more complex cast products.